The pathway leading to the group of antibiotic glycosides called the erythromycins is complex and consists of at least forty enzymatic steps. From preliminary evidence some of the steps are catalyzed by polyenzyme complexes. The approach used in the present study is to define each of the steps in sufficient detail to understand the substrates used, cofactors involved and the nature of the catalytic process. This will probably not involve purification of the enzymes to homogeneity, at least during the initial phase of the study, although each must be understood in some detail. Once this objective is reached, the interaction of the steps and their regulation as well as the relationship of the pathway as a whole to others in the actinomycete (Streptomyces erythreus) will be studied. At present, a lactone synthetase is being defined which presumably makes either the 21-carbon lipid of the antibiotic or some precursor. This synthetase appears to be a polyenzyme complex. The synthesis and attachment of the two sugars (desosamine and either mycarose or cladinose) to form biologically active bis-glycosides is also under investigation. The terminal metabolism of the glycosides, proceeds from erythromycin D either to the B form of the antibiotic or alternately via the C form to erythromycin A, the erythromycin of human medicine. Two hydroxylations catalyzed by cytochrome P-450-linked mixed function oxidases take place and two transmethylation steps involving S-adenosyl-L-methionine are involved in the post lactone synthetase part of the pathway. The product of the lactone synthetase, and erythromycins B and A are potent inhibitors of the pathway.